The present invention relates to a process for treating gases to reduce emissions of oxides of nitrogen using a catalytic composition having a support based on silica and titanium oxide.
It is known that reducing emissions of oxides of nitrogen (NOx) from the exhaust gases of car engines is carried out in particular using three-way catalysts which make stoichiometric use of the reductive gases present in the mixture. Any excess of oxygen is manifested in a drastic deterioration in the performance of the catalyst.
However, certain engines, such as diesel engines or lean-burn petrol engines (i.e. engines operating with a lean mixture) are economic on fuel but emit exhaust gases which permanently include a large excess of oxygenxe2x80x94at least 5%, for example. A standard three-way catalyst therefore has no effect on the NOx emissions of these engines. Nevertheless, limiting NOx emissions has become imperative owing to the more vigorous automotive post-combustion standards which now extend to this type of engine.
There is therefore a real need for an effective catalyst to reduce the NOx emissions for this type of engine and, more generally, for the treatment of gases containing NOx. In addition, the aim is to obtain catalysts which are active at moderate temperature.
With this aim in mind, the process of the invention for treating gases to reduce the emissions of oxides of nitrogen is of the type which uses a catalytic composition comprising a catalytic phase on a support, and is characterized in that the support is based on silica and titanium oxide in an atomic proportion Ti/Ti+Si of between 0.1 and 15%.
Other characteristics, details and advantages of the invention will appear more completely on reading the following description as will various specific but non-limitative examples which are intended to illustrate it.
The principal characteristic of the catalytic composition used in the process of the invention is the support of this composition. This support is based on silica and titanium oxide in the specific proportion given above. In accordance with one embodiment this proportion can be between 1 and 10%.
It is advantageous to use supports which have a higher specific surface area and are thermally stable. It is therefore possible with advantage to use supports having a specific surface area of at least 350 m2/g and more particularly of at least 600 m2/g after calcination at 750xc2x0 C. for 6 hours. By specific surface area here is meant the BET specific surface area determined by nitrogen adsorption in accordance with the standard ASTM D 3663-78 based on the method of Brunauer-Emmett-Teller described in the Journal of the American Chemical Society, 60 (1938) 309.
Supports with these surface values are generally of the mesoporous type; in other words, they have the characteristic of having a significant pore volume contributed by mesopores (pores of between 2 and 10 nm in diameter).
The support based on silica and titanium oxide can be prepared by any process capable of giving a support of sufficient specific surface area.
Mention may thus be made more particularly of a process of micellar texturing using, as silica source, organic compounds of silicium, especially alkyl silicates such as tetraethyl orthosilicate and making this silica source react with a titanium compound or source, in the presence of a surfactant. These organic compounds of silicium are generally employed in the form of solutions in alcohols, especially aliphatic alcohols. As titanium compound, organic titanium compound can be used such as alkyl or alkoxy titanates which are generally employed in the form of alcoholic solutions like the silicium compound. Concerning the surfactant, those of the type with active cation such as amides and quaternary ammonium salts may be employed more particularly. The reaction can be carried out by mixing the organic silicium compound and the titanium compound and by heating. The surfactant is subsequently added to the mixture thus heated. The precipitate obtained is separated from the reaction medium. This precipitate is subsequently calcined, generally in air, to give the support, which can subsequently be shaped. Calcination can be carried out in two stages. In the first stage calcination is carried out at a temperature sufficient to remove the surfactant. This temperature may be approximately 650xc2x0 C. In the second stage, calcination is carried out at a temperature at least equal to that at which the catalyst will be used. This temperature may be approximately 750xc2x0 C.
The support can be present in various shapes, such as pellets, beads, cylinders or honeycombs of variable dimensions.
By way of additives, the support may comprise one or more rare earth metal oxides. By rare earth metals are meant the elements of the group consisting of yttrium and the elements from the Periodic Table whose atomic number is from 57 to 71. As a rare earth metal more particular mention may be made of lanthanum. The additive content, expressed in atomic % of additive/Ti+Si+additive, can be not more than 20%, especially not more than 10%.
The catalytic composition of the invention may additionally comprise a catalytic phase. This phase can be based on at least one metal chosen from the elements of groups IIIA to IIB of the Periodic Table.
More particularly, the catalytic phase is based on at least one metal chosen from the metals of group VIII of the Periodic Table.
The Periodic Table of the Elements to which reference is made is that published in the supplement to the Bulletin de la Socixc3xa9txc3xa9 Chimique de France No. 1 (January 1966).
By way of examples of metals which can be used in the catalytic phase mention may be made of platinum, palladium, rhodium, ruthenium and iridium. Mention may also be made of iron, copper and chromium and of vanadium, niobium, tantalum, molybdenum and tungsten.
The metal content of the catalytic phase and the metal content, especially the platinum content, of the composition can vary within wide proportions. This proportion, expressed as weight of metal relative to weight of support, is commonly between 500 and 40,000 ppm, preferably between 2500 and 20,000 ppm and especially between 5000 and 15,000 ppm.
The catalytic phase can be deposited on the support, preferably the calcined support, by any known technique.
It is therefore possible to use the impregnation technique, by soaking the support in a solution or a sol of the element or elements which makes up or make up the catalytic phase and then removing the excess solution or sol by draining or by passage in a rotary evaporator.
In the particular case of a catalytic phase based on platinum and in accordance with one variant of the invention, the platinum is provided in the form of a sol. The platinum sol will preferably be chosen so that it has a colloidal size of between 2 nm and 10 nm and more particularly between 3 nm and 8 nm. The colloidal size is determined by transmission electron microscopy (TEM).
According to one particular variant, impregnation is carried out xe2x80x9cdryxe2x80x9d; in other words, the total volume of solution or sol used is approximately equal to the total pore volume developed by the support to be impregnated. As far as the determination of this pore volume is concerned, it can be carried out by the known method of mercury porosimetry or else by measuring the quantity of water absorbed by a sample.
Following impregnation, the support is dried if appropriate and then calcined. Drying is usually carried out in air at a temperature which may vary between 80 and 300xc2x0 C. and is preferably chosen between 100 and 150xc2x0 C. Drying is continued until a constant weight is obtained. In general, the duration of drying is between 1 and 24 hours. Calcination of the support with the deposited active or catalytic phase is generally carried out at a temperature of not more than 750xc2x0 C. and, more particularly, not more than 550xc2x0 C. in the case where platinum is used in the catalytic phase. The duration of calcination can vary within wide limits and is, for example, between 1 and 24 hours, preferably between 2 and 10 hours. Calcination is generally carried out in air, although a calcination performed, for example, under inert gas is not of course excluded.
It is also possible to carry out calcination in a water/nitrogen mixture (10% by volume of water in nitrogen, for example). In this case, calcination is carried out at a temperature of not more than 850xc2x0 C. and, preferably, of approximately 750xc2x0 C. This treatment allows the composition to be activated, and is particularly advantageous in the case of a composition whose catalytic phase comprises platinum.
In the particular case of a composition whose catalytic phase is based on platinum provided by a sol, with a mesoporous support and with the specific surface area values given above, the size of the particles of platinum in the composition following calcination is substantially identical to the abovementioned colloidal size.
The catalytic phase can also be deposited by atomization. In this case, the supportxe2x80x94in the form, for example, of a suspensionxe2x80x94is introduced into a solution or a sol of the element or elements which makes up or make up the catalytic phase and the resulting mixture is dried by atomization (spray-drying). It is possible to operate with a gas outlet temperature of between 100 and 150xc2x0 C. Subsequently, calcination is carried out under the conditions described above.
The compositions as described above are employed for the treatment of gases which may comprise oxides of nitrogen in combination, if appropriate, with oxides of carbon and/or hydrocarbons, with a view to reducing, in particular, the emissions of oxides of nitrogen.
Gases suitable for being treated by the present invention are, for example, those obtained from gas turbines, from boilers of thermal power stations or else from internal-combustion engines. In the latter case, these engines can in particular be diesel engines or lean burn engines.
The invention also applies to the treatment of gases which have a high content of oxygen and which include oxides of nitrogen, with a view to reducing the emissions of these oxides. By gases having a high content of oxygen are meant gases having an excess of oxygen relative to the amount required for stoichiometric combustion of the fuels and, more precisely, gases permanently having an excess of oxygen relative to the stoichiometric value xcex=1. The value xcex is correlated with the air/fuel ratio in a manner known per se especially in the field of internal-combustion engines. In other words, the invention applies to the treatment of gases from systems of the type described in the preceding paragraph which function permanently under conditions such that xcex is always strictly greater than 1. In the case of gases having a high content of oxygen the invention thus applies firstly to the treatment of gases from lean burn engines, which have an oxygen content (expressed by volume) of generally between 2.5 and 5% and, secondly, to the treatment of gases which have an even higher content of oxygen, for example gases from diesel-type engines, in other words of at least 5% or more than 5%, more particularly at least 10%, this content being able to be, for example, between 5 and 20%.
The gases may include hydrocarbons and, if so, one of the reactions it is sought to catalyse is the reaction HC (hydrocarbons)+NOx.
The hydrocarbons which can be used as a reducing agent for removing the NOx are, in particular, the gases or liquids from the classes of saturated carbons, ethylenic carbons, acetylenic carbons, aromatic carbons and hydrocarbons from petroleum cuts, such as, for example, methane, ethane, propane, butane, pentane, hexane, ethylene, propylene, acetylene, butadiene, benzene, toluene, xylene, kerosene and gas-oil.
The gases may also include as reducing agent organic compounds containing oxygen. These compounds can be, in particular, alcohols, for example saturated alcohols, such as methanol, ethanol or propanol; ethers, such as methyl ether or ethyl ether; and esters, such as methyl acetate, and ketones.
The invention also applies to the treatment of gases containing neither hydrocarbons nor organic compounds as reducing agent.
The invention also relates to a catalytic system for the treatment of gases with a view to reducing emissions of oxides of nitrogen, which gases may be of the type mentioned previously. This system is characterized in that it comprises a catalytic composition as described above.
In this system, the catalytic composition can be in various forms, such as pellets, beads, cylinders or honeycombs of variable dimensions.
The compositions can also be used in catalytic systems comprising a coating (wash coat) incorporating these compositions, the coating being disposed on a substrate of the metal monolith type, for example, or on a ceramic substrate.
The systems are installed in a known manner in the exhaust pipes of vehicles in the case of their application to the treatment of exhaust gases.
The invention also relates, finally, to the process for producing the abovementioned catalytic systems, employing a catalytic composition of the type described above.
Examples will now be given.